1. Field of the Invention
The present invention relates to an improvement of the Andrussow method for synthesis of hydrogen cyanide (HCN).
2. Description of the Related Art
The synthesis of hydrogen cyanide by the Andrussow method is described in Ullmann""s Encyclopedia of Industrial Chemistry, Volume 8, VCH Verlagsgesellschaft, Weinheim, 1987, pp. 161-162. The educt gas mixture, which comprises methane or a methane-containing natural-gas stream, ammonia and oxygen is passed into a reactor over catalyst gauze and reacted at temperatures of about 1000xc2x0 C. The necessary oxygen is usually introduced in the form of air. The catalyst gauzes comprise platinum or platinum alloys. The composition of the educt gas mixture corresponds approximately to the stoichiometry of the overall equation of the reaction
CH4+NH3+3/2 O2xe2x86x92HCN+3 H2Oxe2x80x83xe2x80x83dHr=xe2x88x92473.9 kJ, 
which takes place exothermically.
The discharged reaction gas contains the product HCN, unreacted NH3 and CH4, as well as important by-products CO, H2, H2O and CO2, and a large proportion of N2.
The reaction gas is cooled rapidly to about 150 to 200xc2x0 C. in a waste-heat recovery boiler and then passed through a scrubbing column, in which the unreacted NH3 is removed with dilute sulfuric acid and some of the steam is condensed. Also known is the absorption of NH3 with sodium hydrogen phosphate solution followed by recycling of the ammonia. HCN is absorbed in cold water in a downstream absorption column and then purified to better than 99.5 wt % by mass in a downstream rectification unit. The HCN-containing water present in the column bottoms is cooled and recycled to the HCN absorption column.
A broad spectrum of possible embodiments of the Andrussow method is described in German Patent 549055.
As an example, a catalyst comprising a plurality of fine-mesh gauze pieces of Pt with 10% rhodium disposed in series is used at temperatures of about 980 to 1050xc2x0 C. The HCN yield is 66.1% based on NH3 used.
A method for maximizing the HCN yield by optimal adjustment of the air/natural gas and air/ammonia ratios is described in U.S. Pat. No. 4,128,622.
In addition to the standard operating procedure with air as the oxygen supply, the use of oxygen-enriched air is described in various documents. Table 1 lists some patents with the operating conditions described therein.
U.S. Pat. No. 5,882,618 describes the synthesis of hydrocyanic acid by the Andrussow method using oxygen-enriched air.
To circumvent the problems that occur under these conditions, such as proximity to the explosion limits of the mixture of NH3, CH4 and oxygen-enriched air, as well as the elevated temperature of the catalyst gauze, which can lead to yield losses and shortened catalyst life, the following measures are proposed:
In a first process step, the system is started up with air as the oxygen source. During this first process step, the catalyst mesh reaches a defined temperature.
In a second process step, oxygen is then metered in and, at the same time, the contents of ammonia and methane are adjusted such that the mixture is situated above the upper explosion limit and the catalyst temperature corresponds to within 50 K of the reference temperature determined in step 1. The temperature of the catalyst gauze is about 1100xc2x0 C. to 1200xc2x0 C.
By means of this procedure, safe use of the system is achieved during operation with oxygen-enriched air.
International Patent WO 97/09273 overcomes the disadvantages of high N2 dilution of the reaction gases by the use of preheated, mixtures of methane, ammonia and oxygen-enriched air or pure oxygen which are capable of detonation.
In order to be able to safely handle the mixtures that are capable of detonation, a special reactor is used that prevents detonation of the reaction mixture. The use of this solution in industrial practice necessitates intensive investment for converting existing HCN plants.
Disadvantages of the Related Art Regarding Operation With Air as the Oxygen Supply
If air is used as the oxygen supply in the starting-gas mixture, the HCN concentration in the reaction gas reaches only about 6 to 8 vol %. Because of establishment of equilibrium, the low HCN concentration in the reaction gas leads to a relatively low HCN concentration of 2 to 3 wt % by mass in the aqueous discharge stream from the sump of the HCN absorber column. Thus, high expenditure of energy is necessary for cooling and separating the large mass flow of absorption water. Furthermore, the high inert-gas content necessitates relatively large apparatus volume and substance streams in the working-up part of the process. Because of the dilution with nitrogen, the hydrogen content in the residual-gas stream is lower than 18 vol %. Thus the hydrogen cannot be economically isolated as a valuable product.
Disadvantages of the Related Art With Oxygen Enrichment in the Starting Gas
The known processes with oxygen enrichment of the educt gas (see Table 1) represent an improvement over the cited disadvantages of operating with air, but they also lead to other limitations. Examples are:
1. If the O2/NH3 or O2/CH4 educt ratios (vol/vol) of the starting gas are not adapted to the degree of enrichment with oxygen, the NH3/CH4/N2/O2 mixture is not sufficiently far from the upper explosion limit, and safe operation of the reactor is no longer assured. Possible consequences are:
(a) danger of explosion
(b) danger of deflagration (damage to the catalyst gauze)
(c) danger of local temperature spikes, which damage the catalyst gauze.
2. The increased oxygen supply to the catalyst leads to increased oxidation of NH3 to N2 and thus to decrease of the HCN yield relative to the feed NH3.
3. In the known processes the degree of enrichment with oxygen is limited to an enrichment of up to about 40% O2 in the oxygen-nitrogen mixture (German Patent 1283209, German Patent 1288575, U.S. Pat. No. 5,882,618).
4. Because of enrichment of the educt gas with oxygen, the catalyst gauze can reach a higher temperature, which leads to faster damage to and deactivation of the catalyst.
5. Possible solutions that counter the existing disadvantages with a specially constructed reactor (International Patent WO 97/09273) require large investments and are not suitable for increasing the performance of existing plants at favorable costs.
One object of the invention was therefore to develop a procedure for performing the Andrussow process for synthesis of hydrogen cyanide with which, by extensive enrichment of the combustion air in existing plants to as much as 100 vol % of oxygen, there are ensured
increased HCN productivity (metric tons of HCN per hour), accompanied by
higher HCN yield relative to feed NH3 and
lower energy consumption per metric ton of HCN as well as
long operating life of the catalyst gauze and
safe plant operation.
These and other objects are achieved by a process for synthesis of hydrogen cyanide comprising reacting methane or methane-containing natural gas, ammonia and oxygen-enriched air or oxygen on a catalyst at an elevated temperature wherein following conditions is satisfied:                     O        2                              O          2                +                  N          2                      =          0.25      -              1.0        ⁢                  xe2x80x83                ⁢                  (                      vol            /            vol                    )                      ,      preferably     greater than           0.40      -              1.0        ⁢                  (                      vol            /            vol                    )                    
and the reaction is carried out in a conventional Andrussow reactor.
Further advantageous embodiments include controlling the molar ratio of                     CH        4                    NH        3              =          0.95      -              1.05        ⁢                  xe2x80x83                ⁢                  (                      mol            /            mol                    )                ⁢                  xe2x80x83                ⁢        in        ⁢                  xe2x80x83                ⁢        the        ⁢                  xe2x80x83                ⁢        educt        ⁢                  xe2x80x83                ⁢        gas        ⁢                  xe2x80x83                ⁢        mixture              ;
intensively mixing oxygen with air to form oxygen-enriched air before adding to the methane or methane-containing natural gas and ammonia; mixing the methane or methane-containing natural gas and ammonia before being metered into the oxygen-enriched air or oxygen; preheating the educt gas mixture prior to reaction, preferably to at most 200xc2x0 C., more preferably to at most 150xc2x0 C.
The disadvantages cited hereinabove of operation with air as the oxidizing agent are avoided by the inventive process. When air is completely replaced by oxygen (O2/O2+N2) molar ratio=1.0), the productivity of existing HCN reactors can be increased by as much as 300% compared with operation with air.
By means of the inventive process, it is surprisingly possible, in addition to the increase in productivity, at the same time to improve the yield of hydrogen cyanide relative to the expensive NH3 raw material.
At the same time, a residual gas with low nitrogen content and thus high calorific value is generated.
Likewise a distinct decrease of the energy consumption per metric ton of produced HCN is achieved by the fact that, by virtue of the greater HCN concentration in the reaction gas, less water has to be circulated for absorption of the formed HCN.
Furthermore, catalyst efficiency (HCN production quantity per kg of catalyst over the entire life of the catalyst) comparable with that for operation with air is achieved.
The cited improvements are achieved with a non-ignitable educt gas mixture, and they ensure safe operation of the reactor.
The degree of enrichment with oxygen can be as high as 100% O2 in the oxygen-nitrogen mixture.